1. Field of the Invention
The present invention relates to a semiconductor device having a circuit including a thin film transistor (hereinafter referred to as a TFT) and a method for manufacturing the semiconductor device. For example, the present invention relates to an electronic device provided with an electro-optical device typified by a liquid crystal display panel or a light-emitting display device including an organic light-emitting element.
Note that the semiconductor devices in this specification refers to all the devices that can operate by using semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor devices.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (having a thickness of approximately several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are widely applied to electronic devices such as an IC and an electro-optical device. In particular, the rapid development of a thin film transistor as a switching element in an image display device is required.
A resistive touch panel or a capacitive touch panel is sometimes attached to the entire display screen of a commercial liquid crystal display device, and it is possible to perform data input using an attached pen or the like.
In addition, a display which includes a photo sensor so that a display screen also serves as an input region has been suggested. Patent Documents 1 and 2 each disclose a display device having an image capture function with the provision of a contact area sensor that captures images. Patent Document 3 discloses a display device provided with a sensor for controlling the luminance of a light-emitting element.
Further, there has been attempted to provide an authentication function to a personal digital assistant such as a mobile phone. A fingerprint, a face, a handprint, a palm print, a hand vein, or the like is used for authentication. When the authentication function is provided for a portion other than a display portion, the number of components may be increased, so that the weight or price of an electronic device might be increased. Moreover, when a fingerprint or the like is detected by a capacitive touch panel, the conductivity may be changed depending on the state of the finger, so that the detection rate might be decreased.